Wiring A Motorcycle From Scratch
Text and Pictures by Mark Trotta
When you're custom wiring a motorcycle, you can start pretty much where you like; the headlight, the taillight, or anywhere in between.
The following procedures should work for any classic bike that is kick-start only and has a battery.
Before you start wiring a motorcycle from scratch, it's best to have all the electrical components on hand. This would include battery, generator, voltage regulator, ignition switch, headlight, taillights, and horn.
Primary Wire
Most old motorcycle wiring is 16-gauge or 18-gauge. Heavier 12-gauge or 14-gauge wire is required when you need something that can handle a bit more load, like starter and generator wires.
The wire rolls you buy in auto parts stores are usually copper wire insulated with plastic. These are SAE-rated, but there is also copper wire insulated with silicone, and higher quality AWG rated wire.
AWG sizing has more copper for a given gauge size than SAE-rated wire does, and more copper is better. This variation is about 10-20%.
Use quality connectors or solder joints to connect wires to terminals. If you choose crimp connectors, take off the colored plastic caps and throw them away. Seal the terminal butts with shrink tubing instead.
Read: Solder or Crimp Motorcycle Wires
Abbreviations for wire colors are usually "BK" for black and "BL" for blue, "R" for red, etc. If the wire colors cannot be duplicated from original colors, make a note for future reference.
Draw Out A Basic Diagram
Drawing out a diagram in a notebook lets you visualize what needs to go where. Start with the ignition switch in the middle of the page. One at a time, add each electrical component to the diagram. Use as many pages as you like.
Mark down where each wire is starting, where it will end up, and what color it will be. I usually reference the factory manual for this.
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Best Place To Start Wiring
For my 1961 Servi-car project, I started with the battery, which was being mounted inside the cargo box.
Battery Size
The first battery I installed was a 12N7-4A battery, which is rated at 7 amp-hours and 74 cold cranking amps. It's a fairly small battery, dimensions are 5-5/16" length x 3" width x 5-1/4" height, and has the vent tube on the positive side.
After the battery kept losing charge from sitting for months at a time, I decided to upgrade to an AGM battery.
The battery now in use is Yuasa YTX16-BS. All AGM batteries are maintenance-free and will hold a charge much longer than a conventional battery. Now the trike can sit for six months and the battery is still completely charged.
Dimensions of the Yuasa YTX16-BS are 6" length, 3-7/16" width, and 6-3/8" height. It's rated at 14 amp-hours and has 230 cold cranking amps.
NOTE: The output of the generator will dictate how big a battery to use and what the amperage should be. Too high an output can cause damage. Not enough amperage will put extra work on the generator.
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Harley Dash and Wiring
J&P Cycles sells a Harley Dash and Wiring Kit that fits many classic Harley Big Twins, including Low Rider, Super Glide, Electra Glide, Wide Glide, and others.
I ended up buying a earlier style two-light dash kit, which is correct for my 1961 Servi-Car. The kit included a speedometer/tachometer, ignition switch, dash lamps, sockets, and hardware.
The hardware including rivets, circuit breaker holders, and a bunch of different shims. I wound up not using most of it.
Curiously, no instructions were included, but since I was custom wiring my bike, it didn't matter.
Five-Post Ignition Switch
Early Harley Big-Twins and Trikes had a five-post switch, which was replaced with a six-post switch sometime in the 1970's. If you have the later six-post and don't need the extra terminal, two of them can be "tied" together with a small jumper wire.
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Circuit Breaker or Inline Fuse?
Installing a 20 amp in-line fuse or circuit breaker from the battery will protect your electrical system in event of an electrical surge. I opted for a 20-amp circuit breaker.
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Wiring Harness
Although wiring harnesses are available for most classic Harleys, if you're wiring a custom bike from scratch, you'll probably need to make a custom wiring harness or two. That 50-year old wiring is probably cracked and frayed anyway, and could use replacing.
There are several styles of wire loom you can use, including black plastic, black cloth, and original-style cloth with patterns. Although original-style cloth looms are most authentic, I bought black cloth looms in 3/16", 1/4", 5/16", and 3/8" rolls.
Shop: Black Cloth Wire Loom
Headlight Wiring
Harley Panheads, Shovelheads, and 1959-up Servi-cars have a large headlamp nacelle which housed the front wiring panel and wires.
Since I was swapping a Wide Glide front end on my Servi, I had to find another way to wire up the headlight. This included wiring the headlight switch inside the handlebars.
Horn Switch
To install a horn button like this you will need to thread-tap two holes. The original Harley machine-screw size 8-32 x 1/4".
Taillight Wiring
Taillights draw three amps or less, so 16-gauge wire is fine.
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Cycle Electric Generator
The single biggest expense of my electrical system (but well worth it) was the Cycle-Electric generator. The voltage regulator is built onto the generator, making it attractive and less cluttered.
Read: Install Cycle Electric Generator
With the built-in regulator, the Cycle Electric generator is a complete 12-volt charging system. It's a simple, two-wire hookup, and gives you higher output and longer service life. It's the last generator you will have to buy for your bike.
Generator Wiring
There are two wires that run from the Cycle-Electric generator, the larger wire goes to the ignition switch. Use a heavy grade wire here. The smaller wire from the A terminal goes to the generator signal light.
After the battery and switch are hooked up, the gen light comes on when you click the key switch to "on" but have not yet started the bike. The light should then go out when the bike starts up. If the light stays on after starting, it's telling you the generator is not charging.
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Distributor and Ignition
I like points ignition for their simplicity and ease of diagnosing. Seems like a lot of modern mechanics don't like points because they don't understand them, and incorrectly assume they are unreliable. Once properly set, a points ignition will run fine for 5,000 miles or more, without needing to be "fiddled" with. So unless your classic bike is your daily driver, points ignition is fine.
Read: Convert Harley To Points Ignition
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6-Volt to 12-Volt Conversion
Harley-Davidson used 6-volt electrical systems on trikes until 1963 and motorcycles until 1964. If you're converting from 6v to 12v, make sure the new battery will fit. You may also need to replace the battery cables.
Ignition points will work with either 6-volts or 12-volts, but the condensers are different. If converting, buy a 12-volt ignition coil. They have more windings and will work much longer.
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Final Thoughts
Once everything is correct and working as it should, use plastic wire ties to secure wiring and harnesses.
Make sure that both the battery and the motor are grounded to the frame.
Wiring a motorcycle from scratch is a challenge. Don't rush. Focus on one thing at a time, then move on to the next. If you break it down into smaller steps, it's easier to do.
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Related Articles
Harley Points And Condenser
Harley Generator - Repair or Replace?
Basic Motorcycle Wiring
How To Wire Handlebars Internally
